In the manufacture of heavy water, i.e. deuterium, the heavy water is extracted from natural water through an exchange with hydrogen sulfide. In a large production facility for the manufacture of heavy water a tremendous collection of pipes and their necessary flanged connections is involved. In fact, in a typical heavy water plant there are over 10,000 flanged connections. In the Canadian environment, two inches or so of insulation is placed over all the piping and their flanges to prevent heat loss. In the event of a leak at a flanged connection, hydrogen sulfide and water will be released to the interior of the insulation where they may combine with oxygen in the air to form both sulfuric and sulfurous acids. Such a combination is highly corrosive and can attack the tie bolts holding the flanges together to the point where an explosive separation could occur, releasing large quantities of hydrogen sulfide. This is of course of great concern to the heavy water industry. Needless to say there are other industries which generate corrosive gases or liquids, the leakage of which is detrimental to the safety of personnel and equipment. It therefore becomes necessary to be able to detect and indicate the presence of corrosive fluids as early as possible so that corrective measures, if required may be carried out.